202 THE FLOWERING PROCESS 



eventually there was a layer of meristematic cells 2 or 3 cells deep 

 covering the entire bud. The bud at this stage is an easily recognizable 

 floral primordium and has passed far beyond the initial changeover 

 from the vegetative to the reproductive state. (3) Enlargement of 

 the pith rib meristem cells caused swelling of the bud. (4) The 

 layer of meristematic cells covering the surface formed the bracts, 

 flower parts, etc. (5) Extension of the axis stopped, and apical 

 dominance is lost. 



An exciting recent report concerns the preliminary findings of 

 E. M. Gilford and H. B. Tepper at Davis, California (see 37). They 

 have initiated histochemical studies on the developing meristem of 

 pigweed {Chenopodium album) and other species. So far they have 

 observed sharp increases in RNA and in protein in the cytoplasm of 

 meristematic cells at the time of transformation to the reproductive 

 condition. It would appear that the genes are busily synthesizing 

 RNA at this time, and that the RNA is in turn causing an increase in 

 enzymes (protein), according to our current concepts of protein 

 synthesis as controlled by messenger RNA from the nucleus. This is 

 the sort of thing one might expect. Transformation should consist of 

 the formation of many new enzymes which are required for floral 

 development. 



Most interesting of all, Gifford and Tepper observed in the nuclei 

 of transforming cells a sharp decrease of the basic protein histone. 

 R. C. Huang and Bonner (52) at Pasadena have recently shown with 

 an in vitro system that DNA fully complexed with histone is completely 

 inactive in synthesizing RNA. Thus it appears that turning on a gene 

 requires removal of histone, which in turn results in production of 

 RNA and then the enzyme controlled by the gene. The observations 

 of Gifford and Tepper seem to indicate that floral transformation 

 consists of turning on nearly all the genes by removing nearly all the 

 histone, which leads to the synthesis of much RNA and many 

 enzymes. Later the histone begins to reappear, indicating perhaps 

 that a more select set of genes is now in control of the situation. Is 

 our flowering hormone simply a remover of meristematic histone ? 



It would seem that we are on the threshold of some exciting 

 discoveries. Such important steps forward could make parts of this 

 discussion obsolete before it can appear in print. We can await rapid 

 advances. 



I 



